Why Are My Raspberries Turning Grey and Mouldy on the Cane?

Raspberries that develop a grey-brown fluffy mould while still attached to the cane — particularly during or after wet weather at harvest time — are infected with Botrytis cinerea, commonly known as grey mould. This is the same fungus that rots strawberries and many other soft fruits. It is ubiquitous in the garden and only becomes a problem when conditions favour it: cool, humid weather, poor air circulation, and damaged or overripe fruit.

Why grey mould appears in summer

Botrytis spores are always present in the garden. They germinate and infect when the conditions align: soft, slightly damaged or overripe fruit tissue, surface moisture (rain or dew), and temperatures between 15°C and 25°C. A wet June or July during raspberry harvest season creates ideal conditions. Berries touching adjacent fruit or foliage, and berries left on the cane past their prime, are the first to be affected. Infection spreads rapidly from one berry to the next when they are in contact.

Harvesting to manage grey mould

Pick every two days rather than once a week — ripe and overripe berries are the entry point for botrytis. Remove any affected berries and debris immediately to reduce the spore load. Do not pick and then leave a bowl of berries at room temperature; refrigerate immediately and use within two days. In a very wet summer, harvesting during a dry period in the morning after dew has dried is preferable to harvesting in wet conditions.

Prevention through cane spacing

The single most effective prevention is correct cane spacing. Thin the fruiting row to six to eight canes per metre after autumn pruning. Well-spaced canes in an open row dry faster after rain, and the improved air circulation dramatically reduces the humid microclimate that botrytis needs. Overcrowded rows are where grey mould is almost always worst.

Fungicide options

For severe or persistent outbreaks, potassium bicarbonate sprays and some copper-based products have activity against botrytis. Apply at flowering to protect the developing berries. However, improved cane spacing and timely harvesting are more consistently effective than spraying alone.